1. Field of the Invention
The present invention generally relates to shunt removal from semiconductor devices and, more particularly, to an improved method for detecting and then removing selected shunt(s) from a semiconductor device such as a photovoltaic cell.
2. Description of Related Art
The interest in photovoltaic (PV) cells has continued as concerns over pollution and limited resources have continued to grow. The continued interest has been in both terrestrial and space applications. In the non-terrestrial environment of outer space, the concern over limited power resources is a major one. This is because the need to increase the amount of power often increases the spacecraft mass. An increased mass can increase the cost of a launch more than linearly due to fuel costs. With the ready availability of solar energy in space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy is an obvious choice. Increasing the efficiency of the solar conversion process can either reduce the spacecraft mass or allow more high value payload to be flown.
Irrespective of the application, and as with any energy generation system, efforts have been ongoing into increasing the output and/or efficiency of PV cells. One such effort to increase efficiency involves converting photon energy from a wider portion of the light energy spectrum. Multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the energy distribution in light. Multiple p-n (or n-p) junctions are formed to create a monolithic stacked multi-junction solar cell.
Efficiency in a monolithic cell PV device can be limited by the existence of defects in the cells themselves. One such defect is a "shunt," which can cause electrical degradation of the cell. A shunt provides a low-resistance path for a portion of the current flow. In a "short," there is virtually no resistance to the current flow. "Shunts" can be caused by foreign material introduced during the growth phase of constructing the PV cell or other type of semiconductor device by metal-organic vapor phase epitaxy (MOVPE) for example.
With the potential for defects to exist in the cell, electrical testing of the device has typically been used to determine if the device can provide useful performance. If several shunts exist, and/or if one significant shunt exists, the electrical degradation may be enough to warrant scrapping the device. In the context of photovoltaic cells, one scrapped cell may represent hundreds of dollars in materials and processing labor. The cost of scrapped cells has continued to increase as the cost of materials used in making the cells has increased. For example, the concern over efficiency in PV cells has created more interest in the use of germanium, gallium arsenide, indium phosphide, and gallium indium phosphide, which tend to be more efficient but also more expensive than their silicon predecessor. Indium phosphide, and phosphide semiconductors in general, have another advantage of being radiation resistant, which is of particular benefit in space applications. Yet, these more advantageous materials can only be expected to increase in cost over the next years.
In an apparent effort to reduce the amount of scrap, application of a reverse bias voltage and/or chemical etching has been used to remove shunts, thereby rendering the device useful. For example, U.S. Pat. No. 4,166,918 discloses a method of improving the performance of an amorphous silicon solar cell by applying a reverse bias current of sufficient magnitude to burn out electrical shorts and shunts, but at less than the breakdown voltage of the solar cell. Chemical etching, however, is not used in this application. One disadvantage to such method is that the application of a reverse bias to a gallium arsenide or similar type cell tends to increase the incidence of shorts or shunts. Another disadvantage is that conditions needed to burn out a severe shunt may damage the cell further.
U.S. Pat. No. 4,543,171 discloses a method of preferentially etching an exposed surface of a photodetector and applying a reverse-bias voltage. In so doing, the temperature of the exposed surface at the defect site increases so that an etchant can remove the defect. A similar method is disclosed in U.S. Pat. No. 4,749,454. The disadvantages in these methods are similar to those described above. Another disadvantage is that a defect may not be located in a position that is susceptible to exposure by the etchant. A further disadvantage is that exposure of the entire device to the etchant may cause damage and electrical degradation to other areas of the cell.
Also described in U.S. Pat. No. 4,543,171 is a method of defect removal that pre-existed such patent. In such pre-existing method, the device is placed under a reverse-bias voltage. The defect is located by means such as thermally sensitive liquid-crystal techniques or infrared-imaging techniques. After the defect is located and the bias removed, an acid etchant is locally applied to the cell to remove the defect. Again, however, a defect may not be located in a position that is susceptible to exposure by the etchant or too much of the cell may be damaged, since the process is not locally selective.
To better appreciate the limitations of the past methods of defect removal, it can be noted that when metal grids are used to collect electrons in a semiconductor, shunts can exist under the metal grids, as well as between them. For a small shunt that exists between metal grids, applying a reverse-bias voltage and/or chemical etchant may be adequate to remove the shunt. But for a shunt that exists under a metal grid (which can be 4 micrometers thick or thicker), applying a reverse-bias voltage of sufficient magnitude to remove semiconductor material (and thereby the shunt) from under the grid would likely degrade the cell further by driving metallic contaminants deeper into the junction, thus causing the cell to be rejected for use. Furthermore, a shunt under a metal grid would not be susceptible to exposure by a chemical etchant.
As can be seen, there is a need for a method of improving the performance of a semiconductor device, including a photovolaic cell, and minimizing the potential for having to scrap such device after it has been grown. Also needed is an improved method of detecting and then removing electrical defects from semiconductor devices, including multi-junction and/or single junction photovoltaic cells. Another need is for an improved method of selectively identifying which of a number of shunts require removal and then removing only those needed to be removed. An additional need is for a method of removing those shunts that exist under a metal grid. Yet a further need is for an improved method of removing shunts that are not susceptible to chemical removal. Also needed is improved apparatus to accomplish the above needs.